Grain Boundary Structure Evolution in Nanocrystalline Al by Nanoindentation Simulations
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Grain Boundary Structure Evolution in Nanocrystalline Al by Nanoindentation Simulations V. Dupont and F. Sansoz School of Engineering, The University of Vermont, Burlington, VT 05405 USA ABSTRACT The nanoindentation of a columnar grain boundary (GB) network in nanocrystalline Al has been examined by atomistic simulation. The goal of this study was to gain fundamental understanding on the relationship between structure evolution at GBs and incipient plasticity for indenter tips significantly larger than the average grain size. The nanoindentation simulations were performed by quasicontinuum method at zero temperature. A GB network made of vicinal and high-angle tilt GBs was produced by generating randomly-oriented 5-nm grains at the surface of a 200 nm-thick film of Al. The major findings of this investigation are that (1) nanocrystalline GB networks profoundly impact on the nanoindentation response and cause significant softening effects at the tip/surface interface; (2) GB movement and deformation twins are found to be the predominant deformation modes in columnar Al, in association with shear band formation by GB sliding and intragranular slip, and crystal growth by grain rotation and coalescence; and (3) the cooperative processes during plastic deformation are dictated by the atomic-level redistribution of principal shear stresses in the material.
INTRODUCTION It has been recently demonstrated that grain boundary (GB) structures have a profound impact on the deformation of nanocrystalline metals. For example, Sansoz and Molinari [1] have predicted by atomistic simulation that the GB behavior of sliding and migration, which are key deformation mechanisms at small grain size, can be controlled by tailoring the GB structural units. Moreover, new evidence of grain growth via GB movement mechanisms has been found in nanocrystalline Al at room temperature under in-situ TEM nanoindentation [2]. Similarly, recent observations of rapid-grain growth at room and cryogenic temperatures have been obtained in nanocrystalline Cu under micro-indentation [3]. However, this atypical behavior of GB structure evolution under indentation is not fully understood at present time. This study intends to provide new insights on the evolution of GB networks in nanocrystalline Al films deformed by nanoindentation using atomistic modeling. Earlier atomistic studies [4-6] have already addressed the nanoindentation of 3D nanocrystalline metal films at zero and finite temperatures. Van Swygenhoven and co-workers [4] have predicted on nanocrystalline Au that GBs can act as source and sink for dislocations nucleated at the indentersubstrate contact, which caused some softening effects as compared to single crystal films. These authors have found that GB sliding and grain rotation occur at very small grain size at finite temperature, but these mechanisms were less easily observed at zero temperature. Other authors [5,6] have also predicted that the elastic limit of thin films can be significantly decreased when indentation occurs o
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